Pivotal bone anchor assembly with snap-in-place bushing having resilient alignment tabs and shank head engaging slots

ABSTRACT

A spinal screw assembly providing an adjustable securement of a fixation rod across at least two vertebrae. The assembly includes a pedicle screw having a spherical head portion, a threaded shaft portion and a tool engagement surface in the head portion for use in driving the screw into a vertebrae. The head portion of the screw is positioned in a body member adjacent a curvilinear surface disposed about an aperture in the end of the body member such that the shaft portion of the screw extends therethrough and the curvilinear inner surface abuts and mates with the head portion of the screw so as to define a ball joint therewith. The body member additionally defines a pair of opposed parallel slots therein adapted to receive a portion of the fixation rod and a locking cap bears against the fixation rod to releasably secure the rod within the assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/452,911, filed Jun. 26, 2019, which is a continuation of U.S. application Ser. No. 16/450,621, filed Jun. 24, 2019, which is a continuation of U.S. application Ser. No. 16/387,201, filed Apr. 17, 2019, which is a continuation of U.S. application Ser. No. 16/386,022, filed Apr. 16, 2019, which is a continuation of U.S. application Ser. No. 16/374,500, filed Apr. 3, 2019, which is a continuation of U.S. application Ser. No. 16/373,054, filed Apr. 2, 2019, which is a continuation of U.S. application Ser. No. 16/372,240, filed Apr. 1, 2019, which is a continuation of U.S. application Ser. No. 14/163,797, filed Jan. 24, 2014, which is a continuation of U.S. application Ser. No. 13/507,857, filed Aug. 2, 2012, now U.S. Pat. No. 8,636,775, which is a continuation of U.S. application Ser. No. 12/154,448, filed May 23, 2008, now U.S. Pat. No. 8,298,265, which is a continuation of U.S. application Ser. No. 10/848,946, filed May 19, 2004, now U.S. Pat. No. 7,377,923, which claims the benefit of U.S. Provisional Application Nos. 60/527,060, filed Dec. 4, 2003, and 60/472,578, filed May 22, 2003, each of which is incorporated by reference in its entirety herein, and for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to an apparatus for internal fixation of the spine and, more specifically to a novel locking mechanism for a variable angle spinal screw assembly that provides for easier implantation, a wide range of motion, ease of disassembly for adjustment or replacement of the stabilization rod and eliminates conventional threaded engagements and the crossover threading, torquing and other problems associated therewith.

Certain spinal conditions, including a fracture of a vertebra and a herniated disc, indicate treatment by spinal immobilization. Several methods of spinal joint immobilization are known, including surgical fusion and the attachment of pins and bone plates to the affected vertebras. One known device is a bone interface anchor inserted into at least two spaced-apart vertebras, with a stabilization rod interconnecting the two or more anchors to stabilize the vertebras spanned by the anchors. Specifically, a bone screw is received within a socket formed in the anchor. The anchor further includes a channel, extending perpendicular to the longitudinal axis of the bone screw, for receiving the stabilization rod. The anchor further comprises a threaded portion above the channel. After the bone screw and anchor have been inserted into the bone material, the rod is placed within the channel and a nut is mated with the external threads of the anchor. The nut applies a compressive force between the rod and the screw head to firmly fix the rod between the spanned vertebras and thus stabilize the spinal vertebrae.

During surgical implantation of these prior art stabilization systems, the surgical site is crowded with tissue masses, sponges and other surgical implements that obstruct access to the anchor threads. Given the difficult access, it is possible for the surgeon to cross-thread the nut with the threads of the anchor after the fixation rod is in place. If the threads of the anchor are cross-threaded, the cross-threaded coupling must be removed and replaced before the surgery can proceed. In addition, the threaded fastener (e.g., the nut) is frequently removed and then reinstalled as the surgeon makes progressive bends to contour the fixation rod. This increases the surgery with each on-off iteration and further increases the chances of cross-threading.

Another problem associated with threaded attachments is the torque exerted on the anchor during the tightening of the threaded fastener about the upper end portion of the fixation device. This torque can inadvertently introduce stress points along the rod, bend the rod or even loosen the threaded engagement of the anchor in the bone. The elimination of the conventional threaded attachments in the fixation device of the present invention also obviates these problems associated with torquing.

The angle at which the anchor screws extend from the vertebra pedicle is dictated by the spinal curvature, the orientation of individual vertebra within the spine, and the surgeon's placement of the screw within the pedicle. For example, there is considerable spinal curvature in the region of the S1-L5 vertebra junction and the angle between the longitudinal axis of the screws and the vertebra in that region vary over a wide range. Also, it may be necessary to displace one or more of the anchors from the spin midline to effectuate maximum spinal stabilization. Thus, the rod-receiving channels are typically not collinear nor coplanar and, the rod must be shaped or contoured by the surgeon during the implantation procedure to fit within the channels along the spinal column. The prior art systems allow the coupling unit to pivot with respect to the screw over a range of about .+−0.20.degree. to .+−0.30.degree., providing some margin for the surgeon to place the rod within the channel.

One challenge with current variable angle or polyaxial systems is aligning the coupling units in a manner that minimizes pre-insertion rod contouring while allowing the surgeon maximum range to optimize pedicle screw placement. This is especially challenging when fusing the S1-L5 junction. The prior art coupling units allow only a limited range of motion with respect to the screw head. The present invention allows a first range of motion in all directions, but also provides an extended range of motion in the medial—lateral—inferior direction (head-to-toe). This extended range of motion, as compared to the prior art, allows the surgeon additional freedom in locating the screws and eases the assembly process by reducing the requirement for rod contouring.

Thus, the present invention provides an extended range of motion as compared to the prior art, allowing the surgeon additional freedom in locating the screws and easing the assembly process by reducing the requirements for rod contouring. The present invention additionally eliminates the numerous problems heretofore experienced with threaded fasteners. The result is a significantly improved variable angle spinal screw assembly.

SUMMARY OF THE INVENTION

Briefly, the present invention comprises a variable angle spinal screw assembly for providing an adjustable securement of a stabilization rod between at least two spaced-apart vertebrae to effect internal fixation of the spine across the affected vertebrae. The assembly is used with at least one other such assembly to secure the fixation rod and includes a pedicle screw, a body member and a locking cap. The pedicle screw has a substantially spherical head portion defining a slot therein used to drive the screw into a vertebrae. The body member is generally cylindrical in configuration and is adapted to receive the head portion of the pedicle screw and cooperate therewith so as to define a modified ball joint to allow variable angular movement of the body member with respect to the pedicle screw with the threaded shaft portion of the screw extending through an opening in the inner end of the body member (or lower end as seen from the perspective shown in the drawings). The body member additionally defines a pair of opposed parallel slots axially disposed in the side wall thereof to receive a portion of the cylindrical fixation rod. The interior walls of the upper portion of the body member are provided with serrations defining a plurality of axially aligned ratchet teeth. The ratchet teeth are adapted to cooperate with opposed mating teeth formed on the outer surface of a locking cap such that upon pressing the locking cap downwardly within the body member of the assembly against the fixation rod and the rod against the head of the pedicle screw, the interlocking teeth will hold the cap in place and secure the fixation rod in place within the assembly. By rotating the locking cap with respect to the body member, the ratchet teeth are disengaged, relieving the pressure of the cap on the fixation rod and thus releasing the securement of the rod.

By providing the body member of the assembly with a rounded interior surface about the lower opening therein that mates with the rounded lower surface of the spherical screw head and with concave exterior surfaces on the underside of the body member about said opening, the angular orientation of the central axis of the body member relative to the pedicle screw is widely variable, providing an extended range of motion to facilitate surgical installation. Through the cooperation of the interlocking ratchet teeth on the body member and locking cap, installation is further facilitated and the disadvantages of conventional threaded fasteners are obviated.

In preferred embodiments of the present invention, a bushing is employed within the body member to better distribute the longitudinal forces exerted on the pedicle screw. The bushing can be of a generally cylindrical configuration, positioned adjacent the interior side wall of the body member and defines a seat for the fixation rod and a bifurcated depending skirt that abuts and mates with portions of the head of the pedicle screw upon being urged thereagainst by the locking cap pressing downwardly on the fixation rod. As a result, the force exerted on the screw is distributed about the head of the screw to improve the locking securement between the screw and the body member.

In addition, by providing a keyed interface between the pedicle screw head and the body member, the pedicle screw can be inserted into the bone by the surgeon unencumbered by the body member. The body member can then be aligned with the head of the embedded screw, slid onto and over the screw head, reoriented so as to mate the inner lower surface of the body member with the screw head to define the above-described modified ball joint and the resulting variable angle or polyaxial relationship. Such a keyed interface can be provided by a threaded engagement between the lower end of the body member and fixed screw head by which the body member can be screwed onto and over the head of the embedded screw. Alternatively, the screw head and body member opening can be multi-sided and configured so as to allow the body member to be slid over the screw head only when the two components are in a given alignment. Once the body member is slid over the head and rotated so as to misalign the respective sides, the body member is locked onto the screw head and the variable angle mating relationship therebetween is formed.

It is the principal object of the present invention to provide an improved securement of a fixation rod between two or more spaced-apart vertebrae to effect internal fixation of the spine across the affected vertebrae.

This and other objects and advantages of the present invention will be readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the variable angle spinal screw assembly of the present invention.

FIG. 2 is a perspective view of the bone screw portion of the assembly of the present invention.

FIGS. 3A and 3B are perspective views of the body member of the assembly of the present invention.

FIG. 4 is another perspective view of the body member of the present invention showing the lower surface thereof.

FIG. 5 is a perspective view of the bushing employed in the present invention.

FIGS. 6A and 6B are perspective views of a first embodiment of the cap of the present invention.

FIG. 7 is a second embodiment of the cap of the present invention.

FIG. 8 is a side view of the variable angle spinal screw assembly of the present invention.

FIG. 9A is a sectional view taken along the line A-A of FIG. 8

FIG. 9B is a sectional view taken along the line B-B of FIG. 8.

FIG. 10 is an exploded view of a modified form of the pedicle screw and body member employed in the present invention.

FIG. 11 is a perspective view of the modified pedicle screw and body member of FIG. 10 shown in the attached position prior to threading the body member over the screw head to form the mating relationship between the spherical lower portion of the screw head and the interior lower surface of the body member.

FIG. 12 is an exploded perspective view of another modified form of the pedicle screw and body member employed in the present invention.

FIG. 13 is a representational side view of the embodiment of the pedicle screw and body member shown in FIG. 12 with the body member on the screw in the mating variable angle position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now in detail to the drawings, the variable angle spinal screw assembly 10 of the present invention comprises a pedicle screw 12, a body member 14, a bushing 16 and a locking cap 18. The assembly 10 is used with at least one other such assembly and a stabilization or fixation rod 19 to connect the assemblies and stabilize the vertebras into which the assemblies are inserted. The pedicle screw 12 preferably employed in assembly 10 has a spherical head 20 defining a slot 22 therein used to drive the screw into the bone. The rounded surface 24 defined by the lower portion of screw head 20 rests upon and mates with a rounded interior surface 26 formed in the inner or lower end of the body member 14 of the assembly 10 so as to form a modified ball joint that provides the desired variable angular movement of the body member with respect to the embedded pedicle screw. The threaded shaft portion 28 of screw 12 extends therefrom through the opening 30 in the lower end of body member 14.

The body member 14 of assembly 10 further defines a pair of opposed parallel slots 32 axially disposed in the side wall 34 thereof, which terminate at their lower ends in curvilinear surfaces 36. The two slots 32 are sized to receive the fixation rod therein as shown in the drawings with the walls 35 defining the slots preferably extending upwardly beyond the midpoint of the rod and can be inclined slightly to provide a slight holding force on the rod prior to securing the rod with the locking cap 18. Thus, during assembly, the surgeon exerts a slight downward force on the rod, snapping the rod into the transverse channel defined by the aligned slots 32.

The outer or upper interior surface of side walls 34 of the body member 14 both have radially projecting serrations formed therein defining a plurality of axially aligned ratchet teeth 38. The exterior bottom surface 40 of body member 14 has spaced outwardly extending concave surface 42 formed therein and a pair of perpendicularly disposed concave surfaces 44. Surfaces 42 and 44, together with mating surfaces 24 and 26 on the screw head and body member of the assembly, provide an extended range of motion of the body member 14 with respect to the pedicle screw 12. In one embodiment, the range of motion is about .+−0.30.degree. in all directions (as measured from the longitudinal axis of the screw) and about .+−0.40.degree. in the inferior-superior direction, the outwardly (as viewed from the screw head) concave surfaces provide the .+−0.40.degree. range of motion, for a total motion range of 80.degree. This extended range of motion, as compared to the prior art, allows the surgeon additional freedom in locating the screws and eases the assembly process by reducing the requirement for a rod contouring.

To secure the fixation rod 19 within the body member 14 of the assembly, locking cap 18 is provided. Cap 18 defines a top portion 48, a pair of opposed arcuate depending leg portions 50 and a centrally disposed depending projection 52 equidistantly spaced from leg portions 50. Central projection 52 preferably defines a planar lower or bottom surface 54. The leg portions 50 of cap 18 each have a plurality of radially projecting serrations formed therein that define a plurality of axially aligned ratchet teeth 56 adopted to engage teeth 38 on the opposed interior side walls 34 of the body member 14, as will be described

A bushing 16 is preferably employed within the body member 14 of the assembly 10 adjacent side walls 34 to better distribute the longitudinal forces exerted on the pedicle screw. Bushing 16 defines a pair of opposed concave surfaces 60 formed in the upper end of a circular skirt 62 so as to define a seat 64 for the fixation rod 19. The lower portion of bushing skirt 62 is slotted at 66 to provide flexibility therein and defines depending tapered end surfaces 68 adapted to abut opposed sides of the rounded screw head 20. A pair of outwardly projecting opposed resilient tabs 70 are provided at the upper ends of the bushing 16 between concave surfaces 60 that in some embodiments are adapted to be received in a snap fitment within a pair of opposed apertures (not shown) formed in the side wall 34 of body member 14 whereupon the rod seat 64 in bushing 16 is aligned with the channel in the body member. Note that in the illustrated embodiment shown in FIG. 3B, for example, the resilient tabs 70 will engage with the body member 14 inner cylindrical surface located below the ratchet teeth 38, the illustrated aperture 72 being located in the vicinity of the ratchet teeth 38 that cooperate with the locking cap 18 and thus at a distance from the bushing 16. Note that only one of apertures 72 is illustrated in FIGS. 3A and 3B to better illustrate the configuration of the ratchet teeth 38. In an alternative embodiment, the tabs could be removed from the bushing 16 and located on the body member 14 for engagement with apertures or other receiving structure or members formed in opposed sides of the bushing.

To provide a basic stability to the system during initial assembly, the bushing 16 with its slotted lower skirt portion can be configured to provide a press fitment about the screw head 20 so that the pedicle screw 12, body member 14 and bushing 16 will not move freely prior to the insertion and securement of the fixation rod. In addition, the upper portion of the bushing could be configured such that the wall surfaces 60 defining the rod seat 64 therein extend upwardly past the midpoint of the rod and are slightly inwardly inclined. This would provide the same slight holding force when the rod is pushed into the bushing seat 64 that was above described with reference to the channel walls 35 in the body member 14 of the assembly 10.

Upon securing the bushing 16 in the body member 14 and the fixation rod 12 in bushing seat 64, the locking cap 18 is positioned such that the depending leg portions 50 thereon are aligned with the side walls 34 of body member 14. Upon pressing the cap 18 downwardly into body member 14, the ratchet teeth 38 and 56 on the assembly body and cap interlock so as to allow the cap to be pressed downwardly but not retracted. As cap 18 is pressed downwardly into the body member of the assembly, the planar bottom surface 54 of the central projection 52 thereon abuts the fixation rod 19 and presses the rod into and against the seat 64 formed on the upper end of bushing 16. The resulting pressure on the bushing causes the tapered surfaces 68 on the lower end of the bushing to press against the rounded surface of the screw head 20, thereby securing the rod in seat 64 and providing a decentralized and evenly distributed force acting along the longitudinal axis of the screw. Thus, the use of bushing 16 creates a taper lock between the pedicle screw and body member and increases the area of contact therebetween. The result is an improved locking securement over that provided by the earlier described direct contact of the fixation rod against the upper end of the screw head.

The interlocked ratchet teeth will allow the surgeon to tighten the clamping force on the fixation rod by simply pressing downwardly on the locking cap 18. The teeth will hold the component parts in place. To adjust or remove the rod 19, the locking cap 18 is simply rotated 90 degrees about its longitudinal axis, whereupon the teeth 38 on the depending leg portions 50 of the cap are aligned with the open slots 32 in the body member 14, allowing the cap to be simply pulled upwardly away from the fixation rod 19. A hexagonally configured slot 71 is provided in the top portion 48 of cap 18 to facilitate the rotation of the locking cap with a suitably sized mating tool.

In use, at least two of the pedicle screws 12 with the body members 14 and attached bushings 16 disposed about the screw are inserted into the vertebra pedicles spanning the vertebra to be fixated. The surgeon preliminary contours the fixation rod and checks the alignment between the rod and the mating channels formed by the slots in the bushing and body member of the assemblies. Since additional contouring is usually required to improve the alignment, the surgeon incrementally adjusts the rod shape and checks the fit within the channels until the rod properly fits in all channels. During the contouring process, a locking cap 18 can be mated with one or more of the body member 14 (by pressing the cap axially into the body member to create the interlock between the ratchet teeth on the body member and the cap) to temporarily hold the rod in place, thereby assisting the surgeon in achieving an accurate fit. The locking caps are then easily removable (by rotating the cap a quarter of a turn to disengage the interlocking teeth), allowing the rod to be further contoured. Once properly contoured, the rod is inserted into the channels and a locking cap is pressed tightly into each body member and bushing to secure the rod in place. To effect securement of the rod at each of the pedicle screw assemblies, it is solely necessary to press the locking cap longitudinally into the body member such that the bottom surface 54 of the central projection 52 on the cap presses against the fixation rod 19, causing the rod to press downwardly against the bushing 16, which in turn mates with and presses against the head of the pedicle screw.

A modified form of the variable angle spinal screw assembly is illustrated in FIGS. 10 and 11. This modified form of the assembly enables the surgeon to insert the pedicle screw in the bone, by itself, unencumbered by the body member. In the prior embodiment, the pedicle screw 12 must be inserted through the body member 14 before the screw can be driven into the bone. With the body member attached, securement of the screw into the bone can be somewhat difficult. In the modified assembly 100, the outer surface of the spherical head portion 120 of the pedicle screw 112 is provided with threads 121, as seen in FIG. 10. As in the prior embodiment, the upper end of head portion 120 is provided with a vertical slot 122 used to drive the screw into place. The lower interior portion 113 of the body member 114 to be used with the modified pedicle screw 112 is provided with threads 115 adapted to engage threads 121 on the screw. As a result, the body member 114 can be threaded onto (see FIG. 11) and over the head 120 of the screw 112 after the screw is driven into place. With the exception of threads 121 and 115, the pedicle screw 112 and body member 114 are identical in configuration to the screw 12 and body member 14 of the prior embodiment. Thus, after the body member 114 is threaded onto and over the screw head and is disposed within the interior of the lower end of body member 114, as seen in FIG. 11, the variable angular relationship therebetween is formed as in the prior embodiment.

A second modified form of the variable angle spinal screw assembly that enables the surgeon to insert the pedicle screw in the bone, by itself, unencumbered by the body member is illustrated in FIGS. 12 and 13. As seen therein, the mating threads on the pedicle screw 112 and body member 114 have been replaced with mating octagonal surfaces. In this second modified assembly 200, the outer surface of the spherical head portion 220 of the screw 212 is provided with an octagonal portion. The octagonal portion is comprised of eight contact surfaces 221, one of which (e.g. 221 a) is unequal in length to the remaining surfaces. The lower interior portion 213 of the body member 214 to be used with the modified pedicle screw 212 is also provided with an octagonally configured portion adapted to engage and mate with the octagonal surfaces on the screw head. Because of the inclusion of a differently-sized surface on both the screw and body member, the pedicle screw 212 will only align with the body member 214 in only one position, i.e., where the shortened contact surface on the screw head is aligned with the correspondingly shortened surface in the lower interior of the body member. Accordingly, the pedicle screw 212 can again be inserted into the bone without being attached to the body member 214. After the screw 212 is driven into place, the body member 214 can be inserted over the screw head with the octagonal surfaces thereon aligned with the corresponding surfaces on the screw head. By pressing the screw body downwardly, it is completely inserted onto the screw head and the mating octagonal surfaces are moved out of engagement. Upon rotating the body member and pulling upwardly on the body member, such that the head is disposed within the interior of the lower end of body member 214 the variable angular relationship therebetween illustrated in FIG. 13 is formed as in the prior embodiments. It is to be understood that this form of the present invention is not limited to the use of mating octagonal surfaces. Any polygonal configuration could be employed on the screw head and body member wherein at least one of the mating surfaces on the screw head and on the body member is correspondingly off-sized or otherwise differently configured from the remaining surfaces on the screw head and body member.

In another embodiment of the invention, the bushing 16 is not employed. The opposed axial slots 32 in the side wall 34 of the body member of the assembly define a seat for the fixation rod 19. When the locking cap is pressed into the body member with the fixation rod extending thereacross, the planar bottom surface 54 of the central projection 52 again abuts the fixation rod and, in this instance, presses the rod against the upper end of the head of the pedicle screw. For such applications, the body member and pedicle screw would be sized such that the upper surface of the screw would project above the bottom of the seat defined by the axially opposed slots 32 so as to enable the rod to press against the screw and create a rigid, yet adjustable, securement between the body member and the pedicle screw. This embodiment can also be utilized with the modified forms of the pedicle screw 128 and body member 114 shown in FIGS. 10 and 11. In all of these embodiments, the components of the variable angle spinal screw assembly are preferably formed of titanium.

It should be noted that while the preferred configuration of the locking cap provides a rounded and flush mounting on the upper ends of the body member 14 when the locking cap is fully inserted against the fixation rod, other locking cap configurations could be employed. For example, FIG. 7 illustrates a locking cap having a generally cylindrical perimeter portion in which the ratchet teeth 56 project radially therefrom along leg portions 50. This configuration is illustrated in FIG. 1. As a result, the upper end of the locking cap would be inwardly offset from the upper end of the body member without adversely effecting the operation of the variable angle spinal screw assembly. Various other changes and modifications also could be made in carrying out the present invention.

Although the present invention has been described by way of exemplary embodiments, it should be understood that many changes and substitutions may be made by those skilled in the art without departing from the spirit and the scope of the present invention, which is defined by the appended claims. 

What is claimed is:
 1. A pivotal bone anchor assembly for securing a fixation rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising: a screw having a longitudinal axis, a head portion with a partial spherical shape defining a hemisphere plane at a maximum width perpendicular to the screw longitudinal axis, a spherical outer surface having a single common radius extending above and below the hemisphere plane, and a threaded shaft portion extending distally from the head portion and configured for attachment to the bone; a body member having a lower portion defining a bottom opening, an upper portion defining an open first channel configured for receiving the fixation rod, and an axial bore centered around a longitudinal axis, the axial bore communicating with a bottom of the body member through the bottom opening and extending upwardly through the open first channel to a top of the body member, the axial bore including at least one internally-accessible downwardly-facing surface between the bottom and the top of the body member, the screw head portion configured for being pivotally retained in the body member lower portion; and a bushing member configured to be top loaded into the axial bore, the bushing member having an open second channel configured for receiving the fixation rod and a pair of outwardly protruding upwardly-facing surfaces configured to be at least partially snapped into an overlapping arrangement under the at least one downwardly-facing surface to inhibit the screw head portion from moving upward within the axial bore, the bushing member including at least two bottom surfaces separated by at least one axially aligned slot and configured to engage the screw head portion without extending below the hemisphere plane of the screw head portion when the screw longitudinal axis and the body member longitudinal axis are co-aligned.
 2. The pivotal bone anchor assembly of claim 1, wherein the bushing member further comprises opposed upright arms defining the open second channel therebetween.
 3. The pivotal bone anchor assembly of claim 2, wherein the bushing member upright arms further include inner wall surfaces that extend upwardly past a midpoint of the fixation rod when the fixation rod is positioned therebetween.
 4. The pivotal bone anchor assembly of claim 3, wherein the inner wall surfaces of the bushing member upright arm are inwardly inclined to hold the fixation rod within the bushing member.
 5. The pivotal bone anchor assembly of claim 1, further comprising the closure, wherein the overlapping arrangement between the pair of outwardly protruding upwardly-facing surfaces of the bushing member and the at least one downwardly-facing surface of the body member does not interfere with a locking function of the closure.
 6. The pivotal bone anchor assembly of claim 1, wherein the bushing member is configured to be positioned within the axial bore after the screw head portion is disposed within the body member lower portion.
 7. The pivotal bone anchor assembly of claim 1, wherein the bushing member further comprises a lower rounded surface that is engageable with the screw head portion.
 8. The pivotal bone anchor assembly of claim 1, wherein the at least one downwardly-facing surface of the body member further comprises a pair of opposed downwardly-facing surfaces.
 9. The pivotal bone anchor assembly of claim 1, wherein the bushing member has a central opening configured to allow passage for a tool to engage the screw head portion.
 10. The pivotal bone anchor assembly of claim 1, wherein the body member upper portion defining the open first channel includes a discontinuous helically wound thread form.
 11. A pivotal bone anchor assembly for securing a fixation rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising: a screw member having a longitudinal axis, a head portion with a partial spherical shape defining a hemisphere plane at a maximum width perpendicular to the screw member longitudinal axis, a spherical outer surface having a single common radius extending above and below the hemisphere plane, and a threaded shaft portion extending distally from the head portion and configured for attachment to the bone; a body member having a lower portion defining a bottom opening, an upper portion defining an open first channel configured for receiving the fixation rod, and an axial bore centered around a longitudinal axis, the axial bore communicating with a bottom of the body member through the bottom opening and extending upwardly through the open first channel to a top of the body member, the body member including at least one downwardly-facing surface that is internally-accessible from the axial bore, the screw member head portion configured for being received and pivotally retained in the body member lower portion; and a bushing member configured to be top loaded into the axial bore, the bushing member having an open second channel configured for receiving the fixation rod and at least one radially outwardly protruding upwardly-facing surface configured to be at least partially snapped under the at least one downwardly-facing surface to inhibit the screw member head portion from moving upward within the axial bore, the bushing member including at least one bottom surface configured to engage the screw member head portion without extending below the hemisphere plane when the screw member longitudinal axis and the body member longitudinal axis are co-aligned.
 12. The pivotal bone anchor assembly of claim 11, wherein the bushing member further comprises opposed upright arms defining the open second channel therebetween.
 13. The pivotal bone anchor assembly of claim 12, wherein the bushing member upright arms further include inner wall surfaces that extend upwardly past a midpoint of the fixation rod when the fixation rod is positioned therebetween.
 14. The pivotal bone anchor assembly of claim 11, wherein the bushing member includes a central opening.
 15. The pivotal bone anchor assembly of claim 11, wherein the at least one bottom surface further comprises a pair of bottom surfaces separated by at least one axially aligned slot.
 16. A pivotal bone anchor assembly for securing a fixation rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising: a screw member having a longitudinal axis, a head portion with a partial spherical shape defining a hemisphere plane at a maximum width perpendicular to the screw member longitudinal axis, a spherical outer surface having a single common radius extending above and below the hemisphere plane, and a threaded shaft portion extending distally from the head portion and configured for attachment to the bone; a body member having a lower portion defining a bottom opening, an upper portion defining an open first channel configured for receiving the fixation rod, and an axial bore centered around a longitudinal axis, the axial bore communicating with a bottom of the body member through the bottom opening and extending upwardly through the open first channel to a top of the body member, the axial bore configured to receive and pivotably retain the screw member head portion, the body member including at least one downwardly-facing surface that is internally accessible from the axial bore; and a bushing member configured for top loading into the axial bore, the bushing member including an open second channel configured for receiving the fixation rod, at least one radially protruding structure configured to hold the second channel in alignment with the first channel, and at least one upwardly-facing surface configured to be resiliently snap under the at least one downwardly-facing surface when the bushing member is downwardly displaced within the axial bore so as to inhibit the bushing member and the screw member head portion from moving upward in the axial bore, the bushing member including a bottom surface with at least one axially aligned through-slot formed therein and configured to engage the screw member head portion so as to not extend below the hemisphere plane when the screw member longitudinal axis and the body member longitudinal axis are co-aligned.
 17. The pivotal bone anchor assembly of claim 16, wherein the screw member head portion further comprises a helically-wound thread on at least a portion of the outer spherical surface extending above and below the hemisphere plane.
 18. The pivotal bone anchor assembly of claim 16, wherein the body member bottom opening further comprises an inner thread.
 19. The pivotal bone anchor assembly of claim 18, wherein the screw member head portion is configured to be bottom loaded into the body member through the bottom opening having the inner thread.
 20. The pivotal bone anchor assembly of claim 16, wherein the at least one axially aligned through-slot further comprises includes a pair of diametrically opposite through- slots extending upward from a bottom edge of the bushing member. 